Combine the sugar and vanilla. In a small bowl, combine the granulated sugar and vanilla bean paste or extract; mash with a spoon until well blended.

Cream the butter and sugars. In a mixing bowl with an electric mixer, beat the butter until smooth and creamy; add the vanilla, granulated sugar mixture, and confectioners' sugar and beat until light and fluffy, about 2 minutes.

Finish the dough. Add the egg yolk and salt to the butter mixture and blend well. Put the egg white in a small bowl or cup and refrigerate it until you are ready to bake. Add the flour and beat on low speed just until blended.

Shape the dough log and chill. Divide the dough into 2 portions and roll each into a log about 9 inches in length. Wrap each roll in plastic wrap and refrigerate for 2 hours or overnight. Hint: cut one side of an empty paper towel roll to keep the rolls from flattening. Place the wrapped log of cookie dough in the roll and tie it loosely with twine. Refrigerate as directed.

Prep the oven and pans. Heat the oven to 350°F and line cookie sheets with parchment paper.

Slice the dough. When you're ready to bake, unwrap a log of cookie dough. Whisk the egg white and brush it over the cookie dough log. Coat the log with sparkling or sanding sugar. Cut to log into 1/2-inch slices and arrange on the prepared baking sheet. Repeat with the remaining log of cookie dough. You should have about 16 to 18 slices.

Bake. Bake the sablé cookies for 14 to 18 minutes, until just lightly browned on the bottoms.